This invention relates to the use of data derived from the pyrolytic oil-productivity index, or POPI, to further predict other characteristics of the oil-bearing reservoir rock and the characteristics of the oil in the reservoir.
A method for characterizing reservoir rock from the pyrolytic analysis of rock samples known as the Pyrolytic Oil-Productivity Index Method, or xe2x80x9cPOPI methodxe2x80x9d, is disclosed in U.S. Pat. No. 5,866,814. The disclosure of U.S. Pat. No. 5,866,814 is incorporated herein in its entirety by reference.
In the practice of the POPI method, the quality of the reservoir rock at a given location and depth is characterized as (a) oil-producing; (b) marginally oil-producing; or (c) non-reservoir or tar occluded. These relative characterizations are based on a comparison of the value of POPIX for a given rock sample X with the value of POPIo that has been previously determined from either (1) oil-stained reservoir rock samples similar to the drilling target that are known to be of good reservoir quality; or (2) a sample of oil that is similar to the expected composition of the well""s target zone. The principal advantage of the POPI method is its ability to provide data in real time based on cutting samples taken from the drill rig, so that on-the-fly changes can be made, e.g., in horizontal drilling directions, to keep the bit in oil-producing reservoir rock. The POPI method can also be used to amass a body of comparative data for a given region or an oil field that can be used in planning further exploration and production.
The analytical procedures for determining the values for POPI are described in U.S. Pat. No. 5,866,814 (Jones and Tobey), and in view of the relationship of the present invention to the POPI method, the following summary is provided to facilitate an understanding of the terminology and significance of the data points.
1. Definitions
As used in this specification and claims, the following terms have the meanings indicated:
HC means hydrocarbons.
ln means natural logarithm.
LV is the weight in milligrams of HC released per gram of rock at the static temperature condition of 180xc2x0 C. (when the crucible is inserted into the pyrolytic chamber) prior to the temperature-programmed pyrolysis of the sample.
TD is the weight in milligrams of HC released per gram of rock at a temperature between 180xc2x0 C. and Tminxc2x0 C.
TC is the weight in mg of HC released per gram of rock at a temperature between Tminxc2x0 C. and 600xc2x0 C.
LV+TD+TC represents total HC vaporizing between 180xc2x0-600xc2x0 C. A low total HC indicates rock of lower porosity or effective porosity. A low value can also indicate zones of water and/or gas.
POPIo is the value of the pyrolytic oil productivity index as calculated for a representative sample of crude oil of the type which is expected to be found in good quality reservoir rock in the region of the drilling and chosen as a standard.
Tmin(xc2x0 C.) is the temperature at which HC volatization is at a minimum between the temperature of maximum HC volatization for TD and TC and is empirically determined for each sample. Alternatively, a temperature of 400xc2x0 C. can be used for samples where there is no discernable minimum between TD and TC. The latter sample types generally have very low total HC yields.
Phi is the average porosity of the rock.
Sxo is the saturation of drilling mud filtrate and represents the amount of HC displaced by the filtrate, and therefore, movable HC.
Phi*Sxo vs depth plotxe2x80x94the area below the curve represents the proportion of porosity which contains movable HC.
Phi vs depth plotxe2x80x94the area between the Phi curve and the Phi*Sxo curve represents immovable HC, or tar.
Gammaxe2x80x94the naturally occurring gamma rays that are given off by various lithologies while measuring directly in the well bore by the prior art petrophysical tools and are reported in standard API (American Petroleum Institute) units.
Caliperxe2x80x94the measured diameter of the well bore taken at the time of running petrophysical logs.
Density porosityxe2x80x94the porosity calculated by prior art methods from the petrophysical bulk density tools using an assumed fluid and grain density.
Neutron porosityxe2x80x94the porosity measured by prior art methods from petrophysical neutron tools.
Deep resistivityxe2x80x94the resistivity measured by deep invasion (long spacing between source and receiver), lateral log or induction petrophysical tools which is used as a measurement of undisturbed formation resistivity.
Medium resistivityxe2x80x94the resistivity measured by medium invasion (medium spacing between source and receiver), lateral log or induction petrophysical tools which is used as a measurement of resistivity of the formation that has been flushed by mud filtrate from the drilling fluid.
Shallow resistivityxe2x80x94the resistivity measured by shallow invasion (short spacing between source and receiver), lateral log or induction petrophysical analytic techniques which is used as a measurement of the resistivity of the mud filtrate from the mud cake that forms on the interior of the well bore during drilling operations.
Neutron-density cross-plot porosity (N-D Phi)xe2x80x94the porosity determined from a common prior art method which compensates for the effects of lithologic and fluid changes that lead to inaccuracies in employing either density or neutron porosity measurements by themselves.
Core plug permeabilityxe2x80x94the permeability measured by prior art methods from cylindrical rock samples that are cut from cores taken from the drilling process that is reported in units of millidarcys (md).
2. Pyrolysis Analytical Procedure
The analytical method used to quantitatively determine the presence of hydrocarbons in reservoir rock samples is known as open-system pyrolysis. In the practice of the POPI method of the invention the following expression is used to provide one or more data points:
ln(LV+TD+TC)xc3x97(TD÷TC)=POPIxe2x80x83xe2x80x83(I)
In the above expression, the term xe2x80x9cln(LV+TD+TC)xe2x80x9d means the natural logarithm of the value and the term xe2x80x9cPOPIxe2x80x9d is used as shorthand for Pyrolytic Oil Productivity Index. The term POPI is also used more broadly hereinafter as a reference to the method of the invention.
In the POPI method for pyrolysis, a time and temperature-programmed instrument heats a small amount of ground rock sample from a starting temperature of 180xc2x0 C. (held for 3 minutes) to 600xc2x0 C. at a rate of increase in temperature of 25xc2x0 C. per minute. During the programmed heating, the hydrocarbons driven from the rock are recorded as a function of temperature. FIG. 1 shows a typical instrument output plot, which is known as a xe2x80x9cpyrogramxe2x80x9d. A typical analysis results in three peaks. The first is composed of hydrocarbons that can be volatized, desorbed, and detected at or below 180xc2x0 C. while the temperature is held constant for the first 3 minutes of the procedure. These are called light volatile hydrocarbons, or xe2x80x9clight volatilesxe2x80x9d (LVHC, or LV). The next phase of the pyrolytic analysis consists of a programmed temperature increase from 180xc2x0 C. to 600xc2x0 C. that usually results in two more distinct peaks. The first of these peaks occurs between 180xc2x0 C. and about 400xc2x0 C., and corresponds to thermal desorption of solvent-extractable bitumen, or the light oil fraction. These are called thermally distilled hydrocarbons (TDHC, or TD). The second peak in this phase (third peak overall) occurs after about 400xc2x0 C., generally after a minimum in pyrolytic yield is observed and extends typically to about 550xc2x0 C. The temperature corresponding to the minimum in pyrolytic yield between TD and TC is referred to as TMIN. This peak is due to the pyrolysis (cracking) of heavier hydrocarbons, or asphaltenes. The materials that thermally crack are called thermally cracked hydrocarbons or xe2x80x9cpyrolyzablesxe2x80x9d (TCHC, or TC).
As will be understood by those familiar with the art, many other types of data are employed in the characterization of reservoir rock and the oil in the reservoir for the purposes of modeling exploration and production. It is therefore an object of the invention to provide improved methods for determining the characteristics of reservoir rock and the oil in the reservoir based on the POPI method.
It is another object of the invention to provide an improved method for determining reservoir rock characteristics relating to water saturation and to API oil gravity that is less expensive, faster and of comparable accuracy to methods known in the prior art.
It is yet another object of the invention to provide an improved method for determining apparent water saturation values (ASw) from preserved core samples and from core samples that have not been specially preserved, and also by a method that is not dependent on data obtained from petrophysical or electric log data.
Another object of the invention is to provide an improved method that will serve as a substitute for the Dean-Stark method and apparatus for estimating water saturation.
Yet another object of the invention is to provide an improved method of determining water saturation that is both qualitative and quantitative and which is superior to the Dean-Stark calculations when the reservoir contains inhomogeneities, light oil, and/or oil-water transition zones.
A further object of the invention is to provide an improved laboratory method that closely matches the water saturation Sw value as determined by calculation from electric log data employing the Archie equation.
A further object of the invention is to provide a method for assessing changes in the saturation and cementation exponents that are required in utilizing the Archie equation.
The above objects and other advantages are achieved by the improvements of the invention in the pyrolytic oil-production index method, or POPI method. In accordance with one aspect of the invention, the numerical values obtained by the application of the prior art POPI method are standardized or normalized.
In another aspect of the invention, the POPI method and associated data are employed in combination with other empirically determined information to provide values of (1) the API gravity for the reservoir oil; (2) the Apparent Water Saturation (ASw) of the reservoir rock; and (3) the cementation and saturation exponents that are used in the Archie equation for calculating the water saturation in the oil-reservoir rock.
The improved method of the invention results in the standardization of the numerical values derived by the POPI method. Applying the normalization or standardization process to the POPI method results in the conversion of the numerical value of POPIo for good oil-producing reservoir rock to a standard value, e.g., 100, which is denominated POPINORM; a sample considered to be non-reservoir rock has a numerical value less than 50 (i.e., less than one-half the original value of POPIo); and sample values from 1/2 POPIo to POPIo that are considered to be marginally productive reservoir rock have corresponding numerical values, i.e., between 50 and 100. This normalization or standardization of POPIo values has several advantages over the method of the prior art, including that of providing a basis for making direct comparisons between and among the indices for different wells and/or regions.
The Pyrolytic Oil-Productivity Index (POPI) method of the prior art is improved in accordance with the invention to utilize a normalized scale, referred to as POPINORM, that is based on a standardized value that indicates good to excellent oil-productivity for the reservoir rock. In a preferred embodiment, the standardized or normalized value is 100. As will be apparent to one of ordinary skill in the art, another value, e.g., 1000, can be used for the purpose of the invention. However, a value of 100 provides a convenient normalization value for the use and analysis of the data. The method comprises the steps of calculating a normalization factor, FNORM, that is applied to the POPI values that were calculated in accordance with the method of the prior art. The improved method for determining cut-off values for POPI renders POPI data easier to compare from well to well and from field to field.
The invention also comprehends three related improved methods that enhance the range of information available for the characterization of oil reservoirs. The first is the method for predicting the API gravity of an oil in a reservoir by direct calculation from a series of POPI measurements on non-preserved cores or cuttings.
The second method of the invention is directed to the calculation of the in-reservoir water saturation (Sw) value from pyrolysis of reservoir rock samples that are derived from either non-preserved cores or fresh rock cuttings recovered at the drill site. The practice of the method is cost-effective, rapid and is not labor intensive, allowing a large number of samples to be processed for an individual well. The end result is an Apparent Water Saturation (ASw) curve produced by direct measurements that can be compared to water saturation (Sw) as calculated by the Archie Equation from indirect down-hole electric log data.
The third improved method is an extension of the determination of the Apparent Water Saturation (ASw). Using the ASw curve, inferred values for the cementation exponent (m) and the saturation exponent (n) are calculated from the electric log data that satisfy the Archie Equation, as well as the pyrolytic data (ASw). The magnitude of the variation of the m and n values compares favorably to the variations present in direct petrophysical measurements on core samples. These values are extremely useful in developing accurate reservoir models, as well as for estimating reservoir reserves. These improved methods have utility as calibration tools for developing additional input data used in reservoir modeling.